The ability to target an alteration of a given gene has become an increasingly powerful investigative tool that helps to define the functional role of genes in development and disease. One of the prototypical uses for gene targeting in the mouse is defining the functional roles of genes involved in cancer. As molecular pathways are further defined the demand for mice with multiple, targeted genetic alterations will also increase. The increasing power of gene targeting must be available to laboratories experienced in animal experimentation as well as those that are not. Knowledge of early mouse development, embryonic stem (ES) cell culture, and chimera production are essential to gene targeting, but remain high-risk technologies that are best provided by skilled core laboratories. The Gene Targeting Service (GTS) concentrates experience in the necessary techniques and quality control to facilitate and expedite successful gene targeting at Yale. By locating this resource in the Section of Comparative Medicine adjacent to the Transgenic Mouse Unit, the GTS complements a highly successful core facility of the Cancer Center that has been serving members for eight years. The GTS offers comprehensive services that include: 1) electroporation of targeting constructs into ES cells, 2) incubation of ES cells in appropriate selection media, 3) isolation of surviving clones and their expansion for subsequent storage in liquid N/2 and for DNA isolation, 4) karyotyping of positive clones that contain the desired gene disruption and 5) injection of ES cells into blastocysts to create chimeric knock-out mice. Cancer Center members can obtain this full line of services for less than $5,000. Ancillary services available through the Yale Animal Resources Center (YARC) facilitate breeding of the chimeric mice to generate homozygosity for the gene disruption. Investigators may choose among any or all of these options. In addition, the GTS scientific staff will advise investigators on construct design and provide cassettes, mouse strain 129/SvDNA and genomic libraries to aid in preparing targeting constructs. By providing flexible comprehensive services, Cancer Center members will benefit from and pay for only those services that answer their specific needs. Additionally, funding for this core resource will enable members of the Cancer Center to obtain these services at a reduced rate.